Influence of atmospheric ammonia on secondary inorganic aerosol formation in PM2.5 during spring 2024 in the Hongseong area, Republic of Korea

Abstract

Nitrate and organic components have been considered key contributors to high PM2.5 concentrations in Korea. This study examines the chemical evolution of aerosol species influenced by atmospheric ammonia (NH3) in Hongseong county, Chungcheongnam-do, Republic of Korea, during late spring 2024 (May 20-June 18), a period characterized by intensive agricultural activity. Using an aerosol chemical speciation monitor and ammonia analyzer, an ammonium-rich atmospheric state (average NH3 concentration: 26.7 +/- 12.4 ppb) was observed, with notable contributions from ammonium (NH4+), nitrate (NO3-), and sulfate (SO42-), especially under high-humidity conditions. The relative contributions of nitrate to PM2.5 for low (0-15 mu g/m(3)), medium (16-30 mu g/m(3)), and high (over 30 mu g/m(3)) concentration intervals were 13.8 %, 23.4 %, and 29.6 %, respectively. Elevated relative humidity (RH), averaging 63.1 % (low), 82.3 % (medium), and 91.7 % (high), played a sig-nificant role in nitrate formation for those concentration intervals. Elevated nighttime ammonium conversion ratios (NHR) highlighted the importance of abundant atmospheric NH3 under high RH conditions, facilitating effective heterogeneous uptake and subsequent particulate ammonium formation. Source tracking using the conditional bivariate probability function (CBPF) model identified agricultural fields, power plants, and indus-trial complexes as major sources of precursor emissions. These results emphasize the need to control emissions of NH3, SO2, and NOx in rural areas to mitigate PM2.5 pollution and improve air quality.